A pressurized water nuclear reactor comprises a vessel containing a core consisting of prismatic fuel arrays arranged side by side and resting vertically on a core-supporting plate which is transverse relative to the vessel.
Another transverse plate, pierced with orifices opposite each of the fuel arrays and referred to as the upper core plate, is arranged above the core.
Tubes for guiding the control rods are arranged on the upper core plate, vertical to some of the arrays.
These vertical tubes, resting with their lower end on the upper core plate, permit the guiding of the control rods, which consist of clusters of long rods containing a strongly neutron-absorbing material.
For the operation of the reactor, these control rods are displaced vertically so as to enter the fuel arrays to a greater or lesser depth, the arrays being equipped with guide tubes substituted for some of the fuel rods in order to receive and guide the absorbing rods of the control rod, inside the fuel arrays.
The guide tubes arranged above the upper core plate are fixed, at their upper part, to a support plate, which is itself joined to the upper core plate via cell-like spacers, which provide the rigidity of the assembly.
The guide tubes, their support plate and the spacers constituting the upper internal equipment of the reactor occupy the upper part of the vessel.
The guide tubes contain discontinuous devices and continuous devices permitting the guiding of the control rod when it is being moved vertically for controlling the reactor, or when it is dropped into the position of maximum insertion for emergency shutdown of the reactor.
The discontinuous guide devices consist of plates located transversely in the tubes, at regular intervals, in the upper part of the latter.
The continuous guide devices consist of long sleeves possessing vertical channels for guiding the control rod, and are located in the lower part of the guide tubes.
To bring the nuclear reactor into operation, all the fuel arrays constituting the core are introduced into the reactor vessel and come to rest on the core-supporting plate; each of the fuel arrays consists of a bundle of fuel rods kept apart by spacers, so that, in transverse planes, the fuel rods form a uniform lattice.
The pressurized water coming into contact with the fuel rods of the arrays serves both as a moderator fluid and a heat-transfer fluid.
In conventional pressurized water nuclear reactors of the moderation ratio in the reactor core, i.e., the ratio of the volume of moderator fluid to the volume of fuel material, is such that the neutrons produced are greatly moderated. Their energy spectrum is said to be "thermal".
By reducing the moderation ratio, it is possible to obtain a different spectrum, called "epithermal", the neutrons being moderated to a much smaller extent.
The transition from one type of operation to another can be achieved by substituting, for fuel arrays of conventional type, arrays which make it possible to obtain a low moderation ratio. To do this, a fuel material enriched in fissile material is used and an arrangement of the fuel rods is adopted which makes it possible to reduce the free volume inside the array. It has been proposed, for example, to use spacers consisting of a simple wire wound in a helix around the fuel rods, making it possible to obtain a low volume ratio of moderator to fuel.
By arranging, in certain parts of the core, in particular at its periphery, fuel arrays containing a fertile material, such as uranium depleted in uranium 235, in a nuclear reactor operating with an epithermal neutron spectrum, it is possible to produce a certain amount of fissile material from the fertile material.